Active anti-islanding system and method

ABSTRACT

An engine control system suited for use with an engine that outputs electrical power to a local load and is electrically connected to an electrical grid. The engine control system includes a set point control operable to set an engine power output value and a sensor operable to measure an electrical parameter between the engine and the electrical grid. A master control system is operable to maintain the engine electrical power at about the engine power output value. The master control system is also operable to vary the engine power output value to maintain the electrical parameter above a predetermined value.

BACKGROUND

The present invention relates to system for inhibiting an islandingcondition from occurring. More particularly, the present inventionrelates to a system and method for inhibiting an islanding conditionfrom occurring in an engine driven generator connected to an electricalgrid.

Microturbine engines are relatively small and efficient sources ofpower. Microturbines can be used to generate electricity and/or to powerauxiliary equipment such as pumps or compressors. When used to generateelectricity, microturbines can be used independent of the utility gridor synchronized to the utility grid. In general, microturbine enginesare limited to applications requiring 2 megawatts (MW) of power or less.However, some applications larger than 2 MWs may utilize a microturbineengine.

In many applications, microturbine engines are used to supply power fora local load that is simultaneously connected to an electrical grid(i.e., a utility grid). When connected to the grid, it is important tomonitor the system for islanding conditions. Islanding exists when themicroturbine engine provides power to the local load with the electricalgrid connection severed at some point. This condition can beproblematic, as the microturbine engine may not be able to react tosudden load changes or may not be able to provide a sudden increase indemand without the support of the electrical grid. As such, anundesirable trip may occur severing all of the power being supplied tothe local load. Therefore, it is desirable to detect islandingconditions and to take steps to isolate the local load or remedy theislanding condition when it is detected.

While many islanding detection schemes are known, most have difficultydetecting islanding conditions that arise when the microturbine outputis closely matched with the local load.

SUMMARY

The present invention generally provides an engine control system suitedfor use with an engine that outputs electrical power to a local load andis electrically connected to an electrical grid. The engine controlsystem includes a set point control operable to set an engine poweroutput value and a sensor operable to measure an electrical parameterbetween the engine and the electrical grid. A master control system isoperable to maintain the engine electrical power at about the enginepower output value. The master control system is also operable to varythe engine power output value to maintain the electrical parameter abovea predetermined value.

In another aspect, the invention generally provides a combustion turbineengine operable to provide electrical power to a local load. The engineincludes a compressor operable to produce a flow of compressed air and acombustor receiving the flow of compressed air and a flow of fuel andproducing a flow of products of combustion. A turbine rotates inresponse to the flow of products of combustion and a generator is drivenby the turbine and is operable to output a quantity of electrical power.The generator includes a first electrical connection to deliverelectrical power to the local load and a second electrical powerconnection that interconnects the generator and the electrical grid. Asensor is positioned to measure an electrical parameter in the secondelectrical connection. A master control system is operable to vary theflow of fuel to the combustor to maintain the quantity of electricaloutput at a preset level. The master control system is also operable tovary the preset level in response to a measured electrical parameterbelow a predetermined value.

In yet another aspect, the present invention generally provides a methodof operating an engine that provides electrical power to a local loadand is electrically connected to an electrical grid. The method includesinputting a total power set point into a master control system andoperating the engine to produce a power output that is substantiallyequal to the total power set point. The method also includes measuringan electrical parameter at a point between the engine and the electricalgrid and changing the total power set point in response to a measuredelectrical parameter below a predetermined value.

In another aspect, the invention generally provides a method ofoperating a power generation unit electrically communicating with a busthat is electrically communicating with a grid and that provideselectrical power to a load. The method includes establishing a minimumpower flow value for power flowing between the grid and the bus. Themethod also includes adjusting the power output of the power generationunit to not match the load and to maintain the absolute value of powerflow between the grid and the bus above the minimum power flow value.

In yet another aspect, the invention generally provides a powergeneration system operable to deliver electrical power to at least oneof a local load and a grid. The system includes a local load bus that iselectrically connected to the grid and provides power to a local load. Aset point control is operable to set a system power output value. Aplurality of engine-generator sets are electrically connected to thelocal load bus. At least one of the plurality of engine-generator setsis operable to deliver a quantity of power to the local load bus. Asensor is operable to measure an electrical parameter between theplurality of engine-generator sets and the grid. A master control systemis operable to maintain the quantity of power generated by the at leastone of the plurality of engine-generator sets at about the system poweroutput value. The master control system is also operable to vary thesystem power output value such that the quantity of power delivered tothe local load bus is not equal to the local load.

BRIEF DESCRIPTION OF THE DRAWINGS

The description particularly refers to the accompanying figures inwhich:

FIG. 1 is a perspective view of a portion of a microturbine engine;

FIG. 2 is a schematic illustration of a portion of a power distributionsystem including the microturbine engine of FIG. 1;

FIG. 3 is a chart illustrating a restricted zone; and

FIG. 4 is a schematic illustration of a portion of a power distributionsystem including a plurality of microturbine engines.

Before any embodiments of the invention are explained, it is to beunderstood that the invention is not limited in its application to thedetails of construction and the arrangements of components set forth inthe following description or illustrated in the following drawings. Theinvention is capable of other embodiments and of being practiced or ofbeing carried out in various ways. Also, it is to be understood that thephraseology and terminology used herein is for the purpose ofdescription and should not be regarded as limiting. The use of“including,” “comprising,” or “having” and variations thereof is meantto encompass the items listed thereafter and equivalence thereof as wellas additional items. The terms “connected,” “coupled,” and “mounted” andvariations thereof are used broadly and encompass direct and indirectconnections, couplings, and mountings. In addition, the terms“connected,” “coupled,” and “mounted” and variations thereof are notrestricted to physical or mechanical connections or couplings.

DETAILED DESCRIPTION

With reference to FIG. 1, a microturbine engine system 10 that includesa turbine section 15, a generator section 20, and a control system 25 isillustrated. The turbine section 15 includes a radial flow turbine 35, acompressor 45, a recuperator 50, a combustor 55, and a gearbox 60.

The engine 10 includes a Brayton cycle combustion turbine with therecuperator 50 added to improve engine efficiency. The engine shown is asingle-spool engine (one set of rotating elements). However, multi-spoolengines are also contemplated by the invention. The compressor 45 is acentrifugal-type compressor having a rotary element that rotates inresponse to operation of the turbine 35. The compressor 45 shown isgenerally a single-stage compressor. However, multi-stage compressorscan be employed where a higher pressure ratio is desired. Alternatively,compressors of different designs (e.g., axial-flow compressors,reciprocating compressors, and the like) can be employed to supplycompressed air for use in the engine 10.

The turbine 35 is a radial flow single-stage turbine having a rotaryelement directly coupled to the rotary element of the compressor 45. Inother constructions, multi-stage turbines or other types of turbines maybe employed. The coupled rotary elements of the turbine 35 and thecompressor 45 engage the gearbox 60 or other speed reducer disposedbetween the turbine section 15 and the generator section 20. In otherconstructions, the coupled rotary elements directly engage the generatorsection 20.

The recuperator 50 includes a heat exchanger employed to transfer heatfrom a hot fluid to the relatively cool compressed air leaving thecompressor 45. One suitable recuperator 50 is described in U.S. Pat. No.5,983,992 fully incorporated herein by reference. The recuperator 50includes a plurality of heat exchange cells stacked on top of oneanother to define flow paths therebetween. The cool compressed air flowswithin the individual cells, while a flow of hot exhaust gas passesbetween the heat exchange cells.

During operation of the microturbine engine system 10, the rotaryelement of the compressor 45 rotates in response to rotation of therotary element of the turbine 35. The compressor 45 draws in atmosphericair and increases its pressure. The high-pressure air exits the aircompressor 45 and flows to the recuperator 50.

The flow of compressed air, now preheated within the recuperator 50,flows to the combustor as a flow of preheated air. The preheated airmixes with a supply of fuel within the combustor 55 and is combusted toproduce a flow of products of combustion. The use of the recuperator 50to preheat the air allows for the use of less fuel to reach the desiredtemperature within the flow of products of combustion, thereby improvingengine efficiency.

The flow of products of combustion enters the turbine 35 and transfersthermal and kinetic energy to the turbine 35. The energy transferresults in rotation of the rotary element of the turbine 35 and a dropin the temperature of the products of combustion. The energy transferallows the turbine 35 to drive both the compressor 45 and the generator20. The products of combustion exit the turbine 35 as a first exhaustgas flow.

In constructions that employ a second turbine, the first turbine 35drives only the compressor, while the second turbine drives thegenerator 20 or any other device to be driven. The second turbinereceives the first exhaust flow, rotates in response to the flow ofexhaust gas therethrough, and discharges a second exhaust flow.

The first exhaust flow, or second exhaust flow in two turbine engines,enters the flow areas between the heat exchange cells of the recuperator50 and transfers excess heat energy to the flow of compressed air. Theexhaust gas then exits the recuperator 50 and is discharged to theatmosphere, processed, or further used as desired (e.g., cogenerationusing a second heat exchanger).

Turning to FIG. 2, a portion of the electrical and control systems of apower distribution system 65 is illustrated schematically. As previouslydescribed, the microturbine engine 10 drives the generator 20 to producean electrical output. The system illustrated herein includes asynchronous generator 20, with other types of generators (e.g.,high-speed alternators, asynchronous generators and the like) alsofunctioning with the present invention. The generator output isdelivered to a local load bus 70 via a generator output line 75. Agenerator sensor 80, positioned within the generator output line 75,measures an electrical parameter of the generator 20 during engineoperation. In most constructions, the generator sensor 80 includes acurrent sensor. The measured current, along with a known voltage, can beused to calculate an actual generator output power. Other constructionscan include multiple sensors that measure current, voltage, and/or powerdirectly. The generator sensor 80 can continuously monitor theelectrical parameter or can take periodic measurements as desired.

The generator output line 75 connects to, and delivers power to, thelocal load bus 70. Various local loads 85 (e.g., motors, computers,monitors, robots, welding machines, lights, etc.) may be powered off thelocal load bus 70.

In some constructions, multiple microturbine engine systems 10, or othergeneration systems (e.g., diesel, solar, wind, fuel cell, and the like)are connected to the local load bus 70, with some or all of themsimultaneously providing power to the power distribution system 65. FIG.4 illustrates one possible system that includes multiple engine systems10. Each engine system 10 is electrically connected to the local loadbus 70 to allow each engine 10 to provide electrical power. Generatorsensors 80 a, 80 b are positioned to measure the actual output of eachengine system 10. The control system 25 is then able to individuallycontrol each engine 10 to produce the desired total output. In manyconstructions, the control system 25 is able to start or stop individualengines to optimize the system's operation, while providing the desiredamount of total power.

A tie line 90 interconnects the local load bus 70 and a utility grid 95.A transformer 100 may be disposed within the tie line 90 to step-up orstep-down the voltage between the utility grid 95 and the local load bus70. The tie line 90 facilitates the delivery of electricity from theutility grid 95 to the local load bus 70 and/or from the microturbineengine 10 to the utility grid 95. The tie line 90 also includes a tieline sensor 105 that measures an electrical parameter (e.g., voltage,current, absolute value of current, power, phase angle, frequency, andthe like). In most constructions, the tie line sensor 105 includes acurrent sensor that measures both the magnitude and direction of currentflow within the tie line 90. However, other constructions may includemultiple sensors that measure current, voltage, and/or power flow. Thetie line sensor 105 can continuously monitor the electrical parameter orcan take periodic measurements as desired.

In some constructions, the microturbine engine 10 operatesintermittently. As such, the tie line 90 is sized to carry sufficientelectricity to power the local loads 85 during periods in which themicroturbine engine 10 is inoperative. With the generator 20synchronized to the utility grid 95 (i.e., the voltage, phase angle, andfrequency of the generator output power matched with the utility gridpower), both the generator 20 and the utility grid 95 can provide powerto the local load bus 70 and the local loads 85.

The microturbine engine 10 includes a control system 25 that controlsthe operation of the engine 10 (or engines in a multiple engine system).The control system 25 manipulates various components (e.g., valves,pumps, compressors, louvers, switches, relays, and the like) thatcontrol various operating parameters of the engine 10. For example, thecontrol system 25 may control fuel flow to the engine 10 to controlengine speed and/or power output. The control system 25 may move orinitiate movement of a controller that in turn may manipulate a valve, acompressor, or other control member to control the flow of fuel to thecombustor 55, which in turn controls the speed or the power output ofthe engine 10.

When the generator 20 is synchronized to the utility grid 95, the speedof the generator 20 is substantially fixed and the control system 25controls output power. A power output set point is supplied to thecontrol system 25, which then maintains the generator output at a valuesubstantially equal to the power output set point. There are manydifferent ways of inputting the power output set point. For example, amanual control could be used. The manual control would allow a user toinput a desired value between the engine's minimum and maximum output.In systems that include more than one engine 10, individual power outputset points for each engine 10 may be used to control the output of eachengine 10. Alternatively, a single power output set point that controlsthe total output of all the engines may be used. In the later case, thecontrol system 25 would determine the specific output levels of eachengine 10 using any one of a number of known schemes. In someconstructions, a preprogrammed curve is used to set the power output setpoint. The curve typically defines a power output set point that varieswith the time of day, day of the week, and/or day of the year. However,other parameters (e.g., temperature, pressure, etc.) could also be usedto vary the power output set point.

During operation of the system 65, it is possible for a failure of theutility grid 95 to occur, thus producing an islanding condition.Islanding conditions can arise at any time, but are particularlyproblematic when the power consumed by the local load 85 is very nearthe power level output by the generator 20. Under these “perfectlymatched” circumstances, the islanding condition is very difficult todetect using known methods (e.g., rate of change of frequency ROCOF, andthe like).

To improve the likelihood of detecting an islanding condition, thepresent system 65 inhibits operation of the engine 10 (or engines) in aparticular range or restricted zone 115. The restricted zone is definedas a power flow within the tie line 90 (into or out of the utility grid95) between zero and a predetermined minimum desired power flow 120,with the minimum desired power flow 120 being a non-zero value.Specifically, the control system 25 receives a signal from the tie linesensor 105 indicating the level of power flow through the tie line 90.The control system 25 then compares that signal to a predetermined valuerepresenting the minimum desired power flow 120 through the tie line 90.The signal may represent a power flow into the local load bus 70 or apower flow to the utility grid 95. If the value falls below the minimumdesired power flow 120, the power output set point is automaticallyadjusted. This process continues until the measured power flow throughthe tie line 90 exceeds the minimum desired power flow 120. With themeasured power flow out of the restricted zone 115, islanding detectionis much easier and is more reliable. As one of ordinary skill willrealize, the actual direction of power flow (i.e. into the local loadbus or out of the local load bus) does not significantly affect theability of the present system to detect islanding so long as sufficientpower is flowing. As such, the absolute value of the measured power inthe tie line 90 is typically all that needs to be measured. In preferredconstructions, a minimum power corresponding to a current flow of 500amps allows for detection of islanding conditions. In still otherconstructions, a power flow of 100 amps or less allows for the detectionof islanding. As one of ordinary skill will realize, the actual minimumdesired power flow may vary greatly depending on the system employed.

For example, in one construction, the power output set point is set at100 W and the minimum desired power flow is set at 0.5 kW. If the localload 85 is 100 kW, the microturbine engine 10 will supply all of thepower to the local load 85 and no power will flow through the tie line90. The control system 25 will detect that the flow through the tie line90 is below the minimum desired power flow 120 and will act to eitherincrease or decrease the power output set point. If the power output setpoint is reduced (to say 99 kW), power will begin flowing (1 kW) intothe local load bus 70 from the utility grid 95. If on the other hand,the power output set point is increased (to say 101 kW), themicroturbine engine output will increase, with the excess power (1 kW)flowing to the utility grid 95. Under either scenario, the absolutevalue of the measured power flow through the tie line 90 will eventuallyexceed the minimum desired power flow 120.

The present system reduces the likelihood of undetected islandingconditions when the engine 10 is driving the generator 20 and producingusable electric power. The generator sensor 80 monitors the current flowfrom the generator 20 and provides feed back to the control system 25.The control system 25 adjusts the engine 10 to match the output powerlevel to the power output set point. The tie line sensor 105 monitorsthe current flow through the tie line 90 and provides an additional feedback loop for the control system 25. The measured power flow at the tieline 90 is compared to the minimum desired power flow 120 and the poweroutput set point is reset if the measured power flow falls below theminimum desired power flow 120. The power output set point can beincreased or decreased as desired to assure that the measured power flowis above the minimum desired power flow 120. It should be noted that theminimum desired power flow 120 can be input by the engine user or can bepreprogrammed into the control system 25. The actual value used is afunction of many variables (e.g., engine size, instrument sensitivity,instrument accuracy, system load variations, and the like). As such, thevalues used herein are exemplary and should not be read as limiting inany way.

The foregoing describes a microturbine engine 10 that drives asynchronous generator 20. As one of ordinary skill will realize, othertypes of generators (e.g., high-speed alternators, asynchronousgenerators and the like) could be used with the present invention.Furthermore, the system has been described as including a single mastercontrol system 25. As one of ordinary skill will realize, the variouscontrol functions could be divided among multiple controllers ormultiple control systems as desired. There is no requirement that asingle control system perform all of the control functions describedherein.

Although the invention has been described in detail with reference tocertain preferred embodiments, variations and modifications exist withinthe scope and spirit of the invention as described and defined in thefollowing claims.

1. An engine control system suited for use with an engine that outputselectrical power to a local load and is electrically connected to anelectrical grid, the engine control system comprising: a set pointcontrol operable to set an engine power output value; a sensor operableto measure an electrical parameter between the engine and the electricalgrid; and a master control system operable to maintain the engineelectrical power at about the engine power output value, the mastercontrol system also operable to vary the engine power output value tomaintain the electrical parameter above a non-zero predetermined value.2. The engine control system of claim 1, wherein the engine includes acombustion turbine that drives a synchronous generator.
 3. The enginecontrol system of claim 1, wherein the sensor is a current sensor andthe electrical parameter is a flow of current between the electricalgrid and the engine.
 4. The engine control system of claim 3, whereinthe flow of current passes in one of a first direction and a seconddirection.
 5. The engine control system of claim 3, wherein the mastercontrol system varies the engine power output value to maintain theabsolute value of the flow of current above 100 amps.
 6. The enginecontrol system of claim 3, wherein the master control system varies theengine power output value to maintain the absolute value of the flow ofcurrent above 500 amps.
 7. A combustion turbine engine operable toprovide electrical power to a local load, the engine comprising: acompressor operable to produce a flow of compressed air; a combustorreceiving the flow of compressed air and a flow of fuel and producing aflow of products of combustion; a turbine rotating in response to theflow of products of combustion; a generator driven by the turbine andoperable to output a quantity of electrical power, the generatorincluding a first electrical connection to deliver electrical power tothe local load and a second electrical power connection thatinterconnects the generator and the electrical grid; a sensor measuringan electrical parameter in the second electrical connection; and amaster control system operable to vary the flow of fuel to the combustorto maintain the quantity of electrical output at a preset level, themaster control system also operable to vary the preset level in responseto a measured electrical parameter below a non-zero predetermined value.8. The combustion turbine engine of claim 7, wherein the firstelectrical connection includes a load bus and the second electricalconnection interconnects the load bus and the electrical grid such thatelectrical power can be delivered from the load bus to the electricalgrid and power can be delivered from the electrical grid to the loadbus.
 9. The combustion turbine engine of claim 8, wherein the generatoris electrically connected to the load bus and the local load iselectrically connected to the load bus.
 10. The combustion turbineengine of claim 7, wherein the generator includes a synchronousgenerator.
 11. The combustion turbine engine of claim 7, wherein thesensor is a current sensor and the electrical parameter is a flow ofcurrent flowing through the second electrical connection.
 12. Thecombustion turbine engine of claim 11, wherein the flow of currentpasses in one of a first direction and a second direction.
 13. Thecombustion turbine engine of claim 11, wherein the master control systemvaries the engine power output value to maintain an absolute value ofthe flow of current above 100 amps.
 14. The combustion turbine engine ofclaim 11, wherein the master control system varies the engine poweroutput value to maintain the absolute value of the flow of current above500 amps.
 15. A method of operating an engine that provides electricalpower to a local load and is electrically connected to an electricalgrid, the method comprising: inputting a total power set point into amaster control system; operating the engine to produce a power outputthat is substantially equal to the total power set point; measuring anelectrical parameter at a point between the engine and the electricalgrid; and changing the total power set point in response to the measuredelectrical parameter below a predetermined non-zero value.
 16. Themethod of claim 15, further comprising periodically varying a loadapplied to the engine.
 17. The method of claim 16, further comprisingmeasuring an electrical parameter between the engine and the local load.18. The method of claim 15, wherein the measuring step includesmeasuring a current flow between the engine and the electrical grid. 19.The method of claim 15, wherein the master control system maintains thepower output of the engine at a value that is substantially equal to thetotal power set point.
 20. The method of claim 15, wherein a mastercontrol system changes the total power set point in response to ameasured electrical parameter below a predetermined value.
 21. A methodof operating a power generation unit electrically communicating with abus that is electrically communicating with a grid and that provideselectrical power to a load, the method comprising: establishing anon-zero minimum power flow value for power flowing between the grid andthe bus; and adjusting the power output of the power generation unit tonot match the load and to maintain the absolute value of power flowbetween the grid and the bus above the minimum power flow value.
 22. Themethod of claim 21, further comprising measuring the power flow betweenthe grid and the bus.
 23. The method of claim 22, wherein the adjustingthe power output of the power generation unit includes adjusting a poweroutput set point in response to the absolute value of the measured powerflow falling below the minimum power flow value.
 24. The method of claim21, further comprising measuring the power output of the powergeneration unit.
 25. A power generation system operable to deliverelectrical power to at least one of a local load and a grid, the systemcomprising: a local load bus electrically connected to the grid andproviding electrical power to a local load; a set point control operableto set a system power output value; a plurality of engine-generator setselectrically connected to the local load bus, at least one of theplurality of engine-generator sets operable to deliver a quantity ofpower to the local load bus; a sensor operable to measure an electricalparameter between the plurality of engine-generator sets and the grid;and a master control system operable to maintain the quantity of powergenerated by the at least one of the plurality of engine-generator setsat about the system power output value, the master control system alsooperable to vary the system power output value such that the quantity ofpower delivered to the local load bus is not equal to the local load.26. The power generation system of claim 25, wherein each of theplurality of engine-generator sets includes a combustion turbine. 27.The power generation system of claim 25, wherein the sensor is a currentsensor and the electrical parameter is a flow of current between thegrid and the local load bus.
 28. The power generation system of claim27, wherein the flow of current is maintained at a non-zero value andpasses in one of a first direction and a second direction.
 29. The powergeneration system of claim 25, wherein the master control system isoperable to initiate and terminate operation of each of the plurality ofengine-generator sets.
 30. The power generation system of claim 25,wherein the master control system establishes a power output set pointfor each of the plurality of engine-generator sets.